Full-Dimensional Quantum Calculations of Vibrational Levels of NH4+ and Isotopomers on An Accurate Ab Initio Potential Energy Surface

Vibrational energy levels of the ammonium cation (NH4+) and its deuterated isotopomers are calculated using a numerically exact kinetic energy operator on a recently developed nine-dimensional permutation invariant semiglobal potential energy surface fitted to a large number of high-level ab initio points. Like CH4, the vibrational levels of NH4+ and ND4+ exhibit a polyad structure, characterized by a collective quantum number P = 2­(v1 + v3) + v2 + v4. The low-lying vibrational levels of all isotopomers are assigned and the agreement with available experimental data is better than 1 cm–1.